Radeon 9000 screen corruption. Or something else?

Hi,
I did a search, but couldn't seem to find a problem like this, so started a new thread. If it's covered elsewhere, then my apologies, and I would appreciate someone pointing me in the right direction.
I've had my system for ages, and the graphics card has always worked a treat, but recently I've been getting strange screen corruption. The corruption is unpredictable, but generally starts about half an hour after the system is turned on. Sometimes it gets gradually worse until the system is completely unusable. At first the corruption can be dealt with by refreshing the screen, but at times it gets so bad that you just can't see what's happening and a refresh makes little difference.
A reboot makes little difference, the corruption will return shortly after a restart. The screen can start showing corruption as soon as the Windows logo shows during reboot, but never while the BIOS is used.
The problem started just after I downloaded and installed the Stardock utility and used it to install Galactic Civilizations II. As you might expect, the first thing I tried was to uninstall Stardock and GalCivII, but the problem persisted. I don't know if the timing was coincidence, but certainly I never had these graphical problems before.
All programs work fine, but all are affected by the corruption.
The corruption usually takes the form of patches of green or magenta colour appearing suddenly in a random area of the screen. Picture below, hope the link works OK.

This is a pretty good example of how bad things can get, though it has been worse at times.

System
As on my user profile, running Win XP Professional. If there's anything else that might be useful then just ask. The graphics card is a Radeon 9000 Excalibur with 64MB in a 4X AGP slot.
The system runs at a good temperature, well within safety limits, though like most AMDs of this age it's fairly hot. I'm not overclocking anything.

Things I've tried:

Update of Catalyst version to 6.11 (I think, though the control centre reports version 1.2.2314.20337).
Update of Direct X to 9.0C.
Update of Sound card drivers.
New installation of Windows on different hard drive with all recent drivers.
Tested on a different monitor.
Tried running with no hardware acceleration.
Tried a full spyware sweep with Spybot, and anti virus and rootkit sweep with AVG (all with latest definitions).
Tried various modes and resolutions with the monitor, no effect.
Tried a thorough dusting of the cards and fans in my system.
Tried reseating the RAM and the Graphics card.
Tried taking the processor down to 1.3 rather than 1.766 Ghtz.
Probably a few more things I don't remember right now...

I'm really stuck with this, I don't have a clue what's causing it, I don't even know if it's actually the graphics card that's the problem. I do know that it's driving me mad, even as I write this. I hope that someone might be able to help out, and any and all help would, of course, be appreciated. Thanks.

I've had this for a while since i got an LCD monitor.
I tried everything untill last night i reinstalled my video driver.I think i had the wrong driver(s) installed
Everything seems ok all of today.
So try removeing and installing the latest drivers.Check for instructions.
I had to remove all entrys in Add/Remove for my card.
I have a Radeon 9600 XT

Zipperman - I believe that the drivers I have are the newest I can get. The latest catalyst version won't install for my card (it's that old), and the ATI driver search points me to the ones I have. I tried an older version of the drivers tonight too, just to check, and the same problem occurred.

Codex - I tried Safe Mode tonight too, and the problem was just as bad. I thought I'd run a virus scan while I was in safe mode, and noticed that the corruption seemed to be shadowing the windows and icons on the screen - a bit wierd:

Rik - Looks like it's possible. In fact I started to look at what's available today, and I'm quite tempted. I've got a friend coming up to see me at the weekend, and he's promised to bring a graphics card with him that I can test in my system. That should tell me finally whether it's the card/drivers or something else.

I'm thinking that perhaps there is a conflict between some of the new software I use and the drivers for the Radeon 9000, the newest version of which was released in Nov 2006. Since ATI don't seem to be doing new drivers for the 9000 series anymore I may not be able to get a fix. Anyway, if I work out anything I'll post it here.

Mine came back so i called the Monitor Maker.
He doesn't feel it's the monitor or the cable.He advised a new video card.
I had planned on 1 anyway.
ATI X1650 Pro AGP.
Or i might go new MBD and a PCIX card.Either at 512 mb.
He said there are many reasons for this and suggested a Pro Repair Shop.
To test all my hardware.
I did some tinkering earlier and all is well right now.Go figure.
I have the Saphire version.I will not buy that again.

I've tested my system with a spare PCI card a friend had, and no problems seemed to exist, so I guess it's either the video card or the AGP slot itself. I believe that I'll try a new graphics card.

I'm thinking of an ATI 9550 with 256 MB. I have one follow up question though:
I have an AGP 4 slot, but all AGP cards now seem to support AGP 8. Are AGP 8 cards backward compatible with AGP 4? I've seen things online that suggest that they are, but thought I may as well ask here before laying out the money.

@ Zipperman - My problems seem to come and go too. I ran the old card for over ten hours yesterday and only needed to refresh the screen two or three times. Tonight I've been online for less than an hour and already I can barely read the screen. It's an infuriatingly inconsistent problem.

I did a compete removal of all Add/Remove ATI entries,
Then used my cd (Saphire) ATI 9600 installation.
Then I updated my Catalyst Control Panel and drivers.This updated NET Framework to 2.0.I think ATI cards need this.Everything seems OK now,except Add/Remove shows My driver as 0 bytes.
I think i'll update to PCIE mbd and Video card. ATI(X1650 Pro) 512
(but not a Saphire version)
RE : "Planning ahead",
I think the 8x factor applies to games that need it.
You have to read your game requirements if you stay with 4X motherboard.